Certain laser processing applications require extremely high throughput processing of a regularly spaced pattern of target locations on a workpiece. For instance, certain solar cell processing applications require the drilling of vias through the silicon wafer in a regularly spaced grid pattern. Customers for these applications require very high processing throughput, on the order of several thousand vias per second.
The spacing of vias in these applications is fairly dense, on the order of 0.5 mm-1 mm. The overall processed area is significant, typically 150 mm×150 mm square wafers. The laser processing system must, therefore, cover this entire area while very rapidly drilling the tight-pitch vias. The accuracy required in such systems is on the order of 10 μm-20 μm. The drill time for each via depends greatly on laser characteristics (wavelength, pulse frequency, pulse power, and pulse width), via diameter, and substrate material and thickness. The drill time is, however, typically on the order of 0.1 msec-0.5 msec. Via diameters are typically on the order of 20 μm-50 μm.
Typical conventional approaches rely on galvanometer (galvo)-based positioning of the laser processing beam, either alone (with a very large galvo field) or combined with a movable stage (with a relatively small galvo field). Each of these approaches has certain limitations.
A first system architecture implementing galvo-based processing laser beam positioning uses a single large galvo field to cover the entire workpiece. This implementation requires either a very large scan lens or a post-lens scanning system. In either case, the galvo typically moves the processing beam at a constant velocity over the entire workpiece, and a controller fires a laser pulse at each via location without stopping the galvo. Several passes are required to fully drill each via. This is possible because of the relatively small number of pulses required for each via and the regularly spaced pattern of target via locations. This approach avoids the timing overhead and thermal effects of frequent galvo acceleration and deceleration, because galvo turnarounds take place only at the edges of the workpiece.
If a very large scan lens is used to cover the entire workpiece field, the large lens is subject to accuracy degradation caused by optics heating that results from working with high-powered laser beams. It also requires a large beam diameter to obtain the required workpiece surface spot size. Such large beam diameters require large galvos, which in turn suffer from accuracy effects resulting from the lower thermal efficiency of moving large (high-inertia) mirrors with large (high-inertia) galvos.
If a post-lens scanning system is used to cover the entire workpiece field, the lens thermal accuracy effects are reduced. The processing system suffers, however, from the effects of non-telecentric beam delivery, which degrades the quality of the drilled vias. Moreover, minimizing such telecentric errors requires that the focal length be kept large, again requiring a large beam diameter to obtain the required workpiece surface spot size. This leads to thermal accuracy issues similar to those described above because of the large galvos required in such systems. If telecentric errors are not of significance, one can use a shorter FL lens and avoid the nonflat focus field problem by using a dynamic focus element. The disadvantages of this approach are cost, complexity, inaccuracy contribution by the focus element; cost of the focus element for very high-speed applications; and residual telecentric error.
A second system architecture is a compound positioning system, in which a small galvo field (typically about 20 mm square) is implemented in conjunction with a structural mechanism that moves a galvo head over the workpiece (either through an X-Y workpiece table, or by a cross-axis moveable optics configuration). As in the first system architecture, the galvo may scan over the vias at a constant velocity, pulsing the processing laser beam at each via, to avoid the overhead of stopping at each via location. As the galvo rapidly scans over its field, the galvo must spend a significant amount of time accelerating and decelerating at the edges of the scan field because it is significantly smaller than the workpiece. This expenditure of time causes a significant reduction in throughput, and if high acceleration is used to reduce the turnaround time, thermal heating of the galvo degrades accuracy and places an upper limit on achievable acceleration. However, the second system architecture does have the advantage of higher accuracy (resulting from reduced lens distortion with the smaller scan lens), improved via quality (resulting from the smaller, lower-distortion scan lens, and the telecentric scan field), and potentially high beam positioning speed (resulting from small galvos and mirrors). Yet this approach may be infeasible because of the throughput limitation described above, depending on the number of laser pulses required to process each via.
What is needed is a laser processing system that can rapidly position the processing laser beam among target locations arranged in a regularly spaced pattern, with acceptable beam quality and precision, at a speed that meets high throughput requirements.